An exhaust gas recirculation system is known in the art, for example, as disclosed in Japanese Patent Publication No. H10-252486, according to which an exhaust port of one of combustion chambers is connected to an intake port of another combustion chamber, so that a part of exhaust gas from the one combustion chamber which is in an exhaust stroke is re-circulated into the other combustion chamber which is in an intake stroke.
According to the prior art having the above structure, flow speed of the exhaust gas swirling in the combustion chamber around a center axis thereof is increased (that is, the swirl of the exhaust gas is increased), because the exhaust gas from the combustion chamber of the exhaust stroke is injected into the combustion chamber of the intake stroke.
Since an exhaust gas purifying apparatus, a muffler, and other devices are generally provided in an exhaust pipe of an engine, pressure in the exhaust pipe between the combustion chambers and those devices is higher than pressure in an intake pipe of the engine. In the multi-cylinder engine, multiple exhaust ports connected to the combustion chambers are converged into the exhaust pipe. Therefore, even when one of the combustion chambers is not in the exhaust stroke, the other combustion chamber is in the exhaust stroke, so that the pressure in the exhaust pipe is always kept at high pressure.
As a result, in the case that one of the combustion chambers in the exhaust stroke is connected to the other combustion chamber in the intake stroke by respective recirculation pipes, the exhaust gas may be re-circulated from the exhaust port to the intake port through the recirculation pipe even during a period in which the other combustion chamber is in strokes other than the intake stroke.
When the exhaust gas is always re-circulated into the intake pipe (namely, re-circulated into the respective intake ports not only in the intake stroke but in the other strokes), amount of the exhaust gas in the intake pipe is increased. As a result, a ratio of intake air among gas to be introduced from the intake pipe into the combustion chamber during the intake stroke is relatively decreased. Ignitionability for the gas (mixture of the intake air, injected fuel, and the exhaust gas) will be adversely affected. Accordingly, it is unavoidable in the prior art to provide a control valve in the recirculation pipe for limiting amount of the exhaust gas to be re-circulated. When such a control valve is provided in the recirculation pipe, sufficient amount of the exhaust gas may not be re-circulated into the combustion chamber in the intake stroke, even in the case that the exhaust gas is forcibly re-circulated into the combustion chamber in the intake stroke by use of the high pressure in the exhaust port for the combustion chamber in the exhaust stroke. This is because the amount of the exhaust gas to be re-circulated through the recirculation pipe is limited by the control valve. As explained above, if large amount of the exhaust gas would be re-circulated into the combustion chamber in the intake stroke, the ignitionability may be deteriorated.
According to the above prior art, the sufficient amount of the exhaust gas could not be re-circulated into the combustion chambers in view of the ignitionability. Therefore, it is not possible to increase the swirl of the exhaust gas in the combustion chambers. It can not be expected in the prior art to facilitate combustion of air-fuel mixture by formation of the swirl of the re-circulated exhaust gas in the combustion chamber.